Gas sensor arrangements comprising a radiation-emitting radiation source, a gas measuring chamber, which may be filled with a test gas containing at least one analyte to be measured, and at least one radiation-detecting detector device which generates an output signal as a function of the presence and/or the concentration of the analyte are known for the detection of a wide variety of analytes, for example methane or carbon dioxide. Conventional gas sensors, as they are disclosed, for example, in EP 0 616 207 A2, WO 00/55603 A1 or DE 199 251 96 C2, are based on the property of many polyatomic gases of absorbing radiation, in particular in the infrared wavelength region. Said absorption occurs in a wavelength characteristic of the respective gas, for example at 4.24 μm for CO2. By means of infrared gas sensors of this type, it is therefore possible to detect the presence of a gas component and/or the concentration of said gas component in a test gas. Such gas sensors comprise a radiation source, an absorption section, i.e. a measuring chamber, and a radiation detector. The radiation intensity measured by the radiation detector is, according to the known Lambert-Beer law, a measure of the concentration of the absorbent gases. In this respect, as in the case of the so-called NDIR (non-dispersive infrared) sensors, a broadband radiation source may be used and a wavelength of interest may be adjusted via an interference filter or grid. Alternatively, a selective radiation source, for example a light-emitting diode or laser, may be used in conjunction with non-wavelength-selective radiation receivers.
Carbon dioxide detection, in particular, is becoming increasingly important in a large number of fields of application. The quality of the interior air, for example, may thus be monitored, the cleaning cycle of self-cleaning ovens may be monitored, the provision of plants with CO2 in greenhouses may be controlled, in the medical field, for example in anaesthetics, the air breathed by a patient may be monitored and wherever there is a risk of CO2 escaping, for example in correspondingly filled air-conditioning systems, a carbon dioxide sensor may be inserted into a warning system.
In order to increase energy efficiency in heating and air conditioning, carbon dioxide detection may be employed in the automotive industry to monitor the CO2 content of the interior air, so as only to initiate a fresh air supply via appropriate ventilation flap control when required, i.e. in the event of an increased CO2 concentration. Moreover, modern motor vehicle air-conditioning systems are based on CO2 as the coolant, so that, in conjunction with escaping CO2 in the event of possible faults, CO2 gas sensors may also fulfill a monitoring function in the automotive industry. In particular in the automotive industry, sensors of this type must satisfy the highest requirements in robustness, reliability and miniaturizability.
A problem occurring especially in the automotive industry is condensation water in absorption gas sensors. As is generally known, condensation water is always produced when air is cooled on surfaces, of which the temperature is lower than the so-called dew point of the air. The dew point depends on the degree of saturation of the air with water and the air temperature.
When absorption gas sensors are used for detecting, for example, CO2 in the interior air of a motor vehicle the problem occurs that, when the internal surface of the wall of the gas measuring chamber exhibits a temperature that is equal to or less than the dew point of the test gas, condensation water condenses on the inner surface of the gas measuring chamber. Said condensation water significantly impairs the measurement process and may also cause damage to the gas sensor arrangement.
An object of the present invention is accordingly to provide a method for measuring by means of a gas sensor arrangement as well as a generic gas sensor arrangement, in which the harmful effects of condensation water may be reduced.